As the integration density of LSIs increases, the circuit line widths of semiconductor devices become narrower year by year. Forming a desired circuit pattern on a semiconductor device involves using a technique in which a high-precision original pattern (also referred to as a mask, or particularly referred to as a reticule if it is used in a stepper or scanner) formed on a quartz member is transferred in a reduced form onto a wafer using a step-and-repeat exposure system. The high-precision original pattern is written by an electron beam writing apparatus using a so-called electron beam lithography technique.
A thermionic-emission electron gun provided with a cathode serving as a heater is employed as an electron gun of an electron beam drawing apparatus. In such an electron gun, electrons are emitted by heating the cathode. The emitted electrons are accelerated with an acceleration voltage and are delivered in the form of an electron beam, which is applied to a sample. In an electron beam drawing apparatus of related art, a current of the electrons emitted from the cathode (i.e., an emission current) is controlled to be held constant.
A variation in the emission current is different depending on a place. For instance, the variation in the emission current from a central portion of the cathode is often different from that in the emission current from an outer peripheral portion of the cathode. Because the electron beam applied to the sample is made up of electrons mainly emitted from the central portion of the cathode, it is preferable that the emission current from the central portion of the cathode is kept constant.
In the past, however, only a total emission current, i.e., the sum of the emission current from the central portion of the cathode and the emission current from the outer peripheral portion of the cathode, is measured, and the total emission current is controlled to be held constant. Thus, the total emission current is controlled to be held at a predetermined value in consideration of that, when the emission current from an outer peripheral portion of the cathode varies, the total emission current also varies in spite of no variation in the emission current from the central portion of the cathode. Such control may vary the emission current from the central portion of the cathode, which has not been varied, and may rather adversely affect drawing accuracy.